Just put an approximate date :)

I can probably do that ;x

Mine was the same as i'm 21 years in so far, i went through all photos i had (physical photos, and some had the date on them) and looked at other items to figure it out. For promotions that took more time as my former professor didn't give out certificates, and was done "old school". I spoke with friends who i trained with and we approximated the belt promotions.